Known devices for delivering aerosol medication for inhalation by a patient include metered dose inhalers that are manually operated and breath actuated. Breath actuated inhalers typically provide a metered dose automatically when the patient's inspiratory effort either moves a mechanical lever or the detected flow rises above a preset threshold, as detected by a hot wire anemometer. See, for example, U.S. Pat. Nos. 3,187,748; 3,565,070; 3,814,297; 3,826,413; 4,592,348; 4,648,393; 4,803,978; 4,896,832; a product available from 3M Healthcare known as Aerosol Sheathed Actuator and Cap; and a product available from Riker Laboratories known as Autohaler. As used herein, references to "effort" and to "flow" are to the movement of air into and out of the patient's pulmonary system. The flow is typically detected as a flow rate (1/min), a flow volume (1), or a combination of a flow rate and flow volume or more than one flow rate and/or more than one flow volume. A major problem with manual metered dose inhalers is that the patient frequently actuates the device at the incorrect time during inspiratory flow, without inhaling, or during expiration and thus does not obtain the benefits of the intended drug therapy. Accordingly, patients may inspire too little medication, or take a second dose and receive too much medication.
One problem with breath activated drug delivery is that the dose is triggered on crossing a fixed threshold inspiratory effort. Thus, an inspiration effort may be sufficient to release a metered dose, but the inspiratory flow following the release may not be sufficient to cause the aerosol medication to pass into the desired portion of the patient's airways. Another problem exists with patients whose inspiratory effort is not sufficient to rise above the threshold to trigger the release valve at all either all of the time or some of the time. This leads to frustration and ineffective therapy.
The known metered dose inhalers include a canister and a body. The canister contains a reservoir of medication and aerosol propellant under pressure, a metering valve which includes a fixed size chamber that captures a defined and uniform volume of material, and a valve stem which operates to release a metered dose. The metering chamber is typically maintained open to the reservoir. To release a dose, the valve stem is pushed into the metering valve. This causes the metering valve, in sequence, to close the chamber relative to the reservoir and capture a fixed volume of material under pressure, to open the chamber relative to the valve stem to release the captured amount of material out a flowpath in the valve stem, to close the chamber relative to the valve stem, and then to open the chamber to the reservoir so that the chamber is positively refilled with medication/aerosol blend under pressure, providing the next dose to be administered.
The metered dose inhaler body contains a receptacle for the canister and a valve actuator (also referred to as a valve stem receptacle) that contains a flow path which terminates in a nozzle and receives the valve stem in alignment with the flow path. The nozzle is typically at an angle to the flow path and directs the released aerosol into the patients' mouth (or nostril). The valve stem receptacle is typically passive. Thus, when the canister is pressed relative to the valve stem receptacle, by manual or automatic advance, the valve stem is pressed into the metering valve and causes the metered dose to be released through the flow paths in the valve stem and the valve stem receptacle and out the nozzle. Typically, the valve stem receptacle has a frictional fit with the valve stem so that the canister is thereby secured to the body.
One problem with conventional pressurized meter dose canister devices is that the metering chamber must be maintained open to the atmosphere for a period of time sufficient to release the entire dosage from the chamber. The required time period is a function of the interior dimensions of the valve stem, the valve stem receptacle, and the nozzle. Consequently, commercial manual metered dose inhalers are limited to valves and medication formulations having release times less than about a tenth of a second. This is so the patient is not burdened with having to control the valve release time in addition to synchronizing the release of medication with inspiration.
Metered dose inhalers also must be sufficiently agitated in order to obtain a homogeneous mixture of the medication and propellant blend for refilling the metering chamber following administration of a dose. A problem with some breath actuated metered dose inhalers is that their operating sequence leaves the metering chamber open to the valve stem and the atmosphere and closed to the reservoir, rather than vice versa. Accordingly, the patient must reset or cock the inhaler to fill the metering chamber with a dose. If this occurs a period of time after release of the last dose, or without sufficiently agitating the device prior to cocking, a non homogeneous mixture of medication and propellant may be loaded into the metering chamber. This results in more or less medication being delivered to the patient than intended.
Another problem with existing metered dose inhalers, whether or not breath actuated, is that the canisters and valve stem receptacles are factory preset to deliver a fixed dose in a relatively short period of time. This results in a given particle size distribution. That distribution may not, however, provide a maximum or optional desired respirable fraction of the aerosol mist that is suitable for a desired location of delivery of the medication in the particular patient. The known devices which attempt to solve this problem process the aerosol after it is generated and thus are inefficient and wasteful. See, e.g., U.S. Pat. Nos. 4,790,305, 4,926,852, 4,677,975 and 3,658,059.
A problem with breath actuated metered dose inhalers that are electronically controlled is that the actuators for pressing the metered dose canister consume considerable amounts of electrical power to deliver the force required to release a dose. Accordingly, they are not practical for use as battery operated devices. See, for example, Newman et al., Thorax, 1981, 36:52-55; Newman et al., Thorax, 1980 35:234; Newman et al., Eur. J. Respir. Dis., 1981, 62:3-21; and Newman et al., Am. Rev. Respir. Dis., 1981, 124:317-320 (the "Newman references").
It is well known that pulmonary functions, such as forced expiratory volume in one second, forced vital capacity, and peak expiratory flow rate, can be measured based on measured flow rates and used both to diagnose the existence of medical conditions, to prescribe medication, and to ascertain the efficiency of a drug therapy program. See, for example, U.S. Pat. Nos. 3,991,304 and 4,852,582 and the Newman references. Heretofore, these tests have been performed using available spirometers. U.S. Pat. No. 4,852,582 also refers to using a peak flow rate meter to measure changes in peak flow rate before and after administration of a bronchodilator. The results of such tests before and after administration of several different medications are used to evaluate the efficacy of the medications.
A problem with the foregoing pulmonary function test devices is that they are complicated. Another problem is that the test data must be examined and interpreted by a trained medical practitioner to be meaningful. Another problem is that they do not provide adequately for altering the dosage of the medication administered in a single patient during the course of therapy, or from patient to patient, using the same delivery device for generating an aerosol of the same or different medications.
Another problem with the known techniques is that they do not meet the needs for a portable device that is hand held, battery powered, and measures flow in two directions such that each direction has a different range of flow values with good resolution in each range.